1. Field of the Invention
The present invention relates to a data demodulator, and more particularly, to a data demodulator for demodulating the radio data system (RDS) broadcast operable in Europe.
2. Description of the Related Art
The Auto-fahrer Rundfunk Informations (ARI) broadcast system is popularized as one of the information providing services capable of mitigating traffic jam problems in Europe. In the ARI broadcast system, a broadcast station for broadcasting the road traffic information multiplexes a subcarrier having a frequency of 57 kHz, i.e., an xe2x80x9cSK signal,xe2x80x9d onto a speech signal. A receiver including a detection unit can recognizing this SK signal. This detection unit can detect as to whether or not a traffic information broadcasting program can be received from the presently tuned broadcast station based upon this SK signal detection result.
Furthermore, the amplitude of this subcarrier is modulated by using a specific frequency. The receiver can recognize that broadcasting of the regional information and the traffic information is commenced or finished by detecting this specific frequency. The signal regarding the regional information is referred to as the xe2x80x9cBK signal,xe2x80x9d and the signal regarding the start/end of the traffic information is referred to as the xe2x80x9cDK signal.xe2x80x9d The combination of the SK signal, BK signal, and DK signal is called the ARI modulation signal.
The RDS broadcast system is also known in this field. The RDS broadcast system is further developed from the above-explained ARI broadcast system, and is capable of providing various information services in the format of digital data. The technical specification of the RDS broadcast system is standardized by European Broadcasting Union (E.B.U.). On the transmission side, the transmission data is differentially encoded, and then a clock signal having the frequency of 1.1875 kHz is modulated in a 2-phase PSK modulation manner by using the differentially-encoded signal. Furthermore, the amplitude of the 57 kHz signal corresponding to the subcarrier is modulated in a subcarrier suppression type amplitude modulation manner by using this 2-phase PSK modulation signal. Then, a double-side-band (DSB) signal is multiplexed onto a speech signal. This double-side-band signal is referred to as an xe2x80x9cRDS modulation signal.xe2x80x9d
A receiver demodulates the DSB signal transmitted in accordance with the above-described technical specification, and is synchronized with the data in accordance with rules of E.B.U., so that the receiver can decode the message. It should be noted that the subcarrier of the RDS modulation signal has an in-phase relationship, or a quadrature-phase relationship with the third higher harmonic wave of the pilot signal (19 kHz) indicative of the stereophonic broadcasting program.
Both the RDS signal and the ARI signal can be simultaneously transmitted. For such a simultaneous transmission, the respective subcarriers are set to the same frequency of 57 kHz, and the quadrature-phase relationship can be continuously established between the phases of these carriers. The frequency shift of the RDS modulation signal with respect to the main carrier is usually +2 kHz to xe2x88x922 kHz However, in the case that both the RDS modulation signal and the ARI modulation signal are transmitted at the same time, the frequency shift of the RDS modulation signal with respect to the main carrier is set to +1.2 kHz to xe2x88x921.2 kHz, whereas the frequency shift of the ARI signal with respect to the main carriers set to +3.5 kHz to xe2x88x923.5 kHz.
In FIG. 3, there is shown a spectrum of an RDS modulation signal 2 and a spectrum of an ARI modulation signal 3, which are multiplexed on a speech signal 1. To recognize such an RDS modulation signal on a.receiver side, a demodulator designed for this specific purpose is required. This demodulator will now be explained with reference to FIG. 4 which shows a schematic block diagram of a conventional RDS data demodulator. This conventional RDS data demodulator includes a filter means 4 and an RDS demodulating means 5. The filter means 4 extracts an RDS modulation signal 7 from an analog FM demodulation signal 6 which is demodulated by using the analog signal processing technique. The RDS modulation signal 7 is outputted from the filter means 4. The RDS demodulating means 5 demodulates this output signal from the filter means 4 to derive an RDS data signal and a reproduction clock signal used to demodulate the RDS data.
In general, the filter means 4 employs an analog filter such as a switched capacitor circuit. At the output terminal of this filter means 4, the RDS modulation signal 7 which has been separated from the speech (audio) signal is outputted. It should also be understood that when the RDS modulation signal and the ARI modulation signal are simultaneously broadcasted from the broadcast station, both the RDS modulation signal and the ARI modulation signal are outputted at the same time.
Both the extracted RDS modulation signal and the extracted ARI modulation signal are supplied to the RDS demodulating means 5. The RDS demodulating means 5 contains a costas loop type PLL for demodulating the DSB signal. As shown in FIG. 5, the costas loop type PLL includes multipliers 8 and 9, a phase comparator 10, a loop filter 11, and a VCO 12. This type of PLL circuit carries out synchronization even when there is no subcarrier. That is, a synchronization can be established when the subcarrier becomes 0 degree, or 90 degrees with respect to the VCO. Consequently, such a PLL circuit is suitable for demodulating an RDS modulation signal having no subcarrier.
In the above-explained conventional RDS data demodulator, if only a RDS modulation signal is transmitted, the RDS modulation signal, which has been DSB-demodulated, is outputted as the synchronization-detection output 13. If both the RDS modulation signal and the ARI modulation signal are transmitted at the same time, such an RDS modulation signal, which has been DSB-demodulated, is outputted as the quadrature detection output 14. This is because when both the ARI modulation signal and the RDS modulation signal are transmitted at the same time, only the ARI modulation signal is synchronized since the modulation factor of the ARI modulation signal is higher than that of the RDS modulation signal. As a result, the RDS modulation signal having the quadrature-relationship with the ARI modulation signal is outputted as the quadrature modulation output 14.
Accordingly, when the costas loop type PLL circuit is used, one has to switch the ARI modulation signal and the RDS modulation signal in order to deal with simultaneous transmission of the ARI modulation signal and the RDS modulation signal. Japanese Unexamined Patent Publication No. 62-206929 discloses an improved method capable of switching the ARI modulation signal and the RDS modulation signal. As shown in FIG. 5, in a method disclosed in the above-referenced Patent Publication, an ARI signal detecting circuit 15 is provided to receive the synchronization-detection output 13 of the costas loop type PLL circuit for judging whether or not the ARI signal is present. Furthermore, a signal switching circuit 16 is employed to select between the synchronization-detection output 13 and the quadrature-detection output 14. In response to a judgment result made by the ARI signal detecting circuit 15, either the synchronization-detection output 13 or the quadrature-detection output 14 is outputted from the signal switching circuit 16 to a post-stage circuit (not shown), so that the RDS signal which has been DSB demodulated is derived.
However, this conventional circuit arrangement has the following problems. When both the RDS modulation signal and the ARI modulation signal are transmitted at the same time, the RDS signal cannot be derived until the ARI signal is detected by the ARI signal detecting circuit 15. Therefore, a lengthy time period is required to obtain the RDS data.
In addition, even when only the RDS modulation signal is transmitted, the above-explained costas loop type PLL circuit would be locked to a third higher harmonic wave. This third higher harmonic wave is produced when the pilot signal having the frequency of 19 kHz and indicative of the stereophonic broadcasting program is distorted due to a multi-path phenomenon. Therefore, the ARI signal detecting circuit 15 erroneously detects an ARI signal. As a result, the signal switching circuit 16 makes the wrong selection in accordance with the wrong result from the ARI signal detecting circuit 15.
As seen from the above descriptions, it would be desirable to separate the ARI modulation signal from the RDS modulation signal before the signals enter the RDS demodulating means 5. It would also be desirable to provide only the RDS modulation signal to the RDS demodulating means 5 for obtaining the RDS data.
As one example of methods capable of separating the ARI modulation signal from the RDS modulation signal, the technical publication xe2x80x9cDesign principles for VHF/FM radio receivers using the EBU radioandta system RDSxe2x80x9d of E.B.U. has proposed a filter means including a delay circuit with a CCD (charge-coupled device). Such a filter means is capable of attenuating the ARI modulation signal.
However, as illustrated in the spectrum in FIG. 3, since the ARI modulation signal is located very close to the RDS modulation signal, a filter having a high Q is required in order to attenuate only the ARI modulation signal. Thus, the above-proposed method has a problem in that its pass-band blocking frequency fluctuates due to the circuit elements used. Also, the size of the circuit is increased. Consequently, such a filter means is not appropriate for mass production.
Accordingly, the present invention is directed to a RDS data demodulator that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an RDS data demodulator capable of attenuating an ARI signal in high precision without increasing a circuit size.
Another object of the present invention is to provide an RDS data demodulator capable of acquiring RDS data continuously under stable condition irrespective of presence or absence of an ARI modulation signal. The RDS data demodulator of the present invention no longer uses the ARI signal detecting circuit and the signal switching circuit, which are employed in the conventional RDS data demodulator.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, an RDS data demodulator of the present invention includes an analog-to-digital converter for converting an analog FM signal into a digital FM modulation signal; a first filter to which the digital FM modulation signal is supplied, having a transfer zero point at a predetermined frequency, and for attenuating an information modulation signal; a second filter to which a filter output signal of the first filter is supplied, having a pass band characteristic at the predetermined frequency, and for extracting an RDS modulation signal; and RDS demodulating means for demodulating a filter output signal of the second filter so as to output both an RDS data signal and a reproduction clock signal used to demodulate the RDS data.
According to a second aspect of the present invention, in the RDS data demodulator described above, a signal processing time period of the first filter is carried out at a frequency higher than that of a subcarrier of the RDS signal by four times; and a term xe2x80x9cZxe2x88x921xe2x80x9d of the denominator of a transfer function of the first filter is equal to zero.
According to a third aspect of the present invention, in the RDS data demodulator described above, a signal processing time period of the second filter, instead of the first filter, is carried out at a frequency higher than that of a subcarrier of the RDS signal by four times; and a term xe2x80x9cZxe2x88x921xe2x80x9d of the denominator of a transfer function of the second filter is equal to zero.
The RDS data demodulator of the present invention operates as follows. In this RDS data demodulator, an analog FM demodulation signal demodulated by way of an analog signal processing technique is converted into a digital FM demodulation signal 19 by an analog-to-digital (A/D) converter 18 for converting the analog FM demodulation signal into the corresponding digital FM demodulation signal. A signal 21 with an ARI modulation signal attenuated from the digital FM modulation signal 19 is produced from a first infinite impulse response type filter 20. This first infinite impulse response type filter 20 has a transmission zero point at a frequency of 57 kHz, and is capable of attenuating the ARI modulation signal. Then, a this signal 21 with the ARI modulation signal attenuated is inputted to a second infinite impulse response type filter 22. This second infinite impulse response type filter 22 has a pass band characteristic at a frequency of 57 kHz, and is capable of extracting an RDS modulation signal. The sufficiently attenuated ARI modulation signal is then supplied to the RDS demodulating circuit 24. Consequently, even when both the RDS modulation signal and the ARI modulation, signal are transmitted at the same time, the ARI signal is no longer detected. Furthermore, the RDS modulation signal is no longer adversely influenced by the noise. As a result, the demodulated signal is more reliable.
Also, since the infinite impulse response type filter is used, such a filter having a high Q is capable of attenuating only the ARI modulation signal. Moreover, the pass-band blocking frequency is not fluctuated due to the circuit elements used. Also, the circuit size is small.
In addition, since the signal processing time period of the first infinite impulse response type filter is selected so that its frequency is higher than that of the subcarrier of the RDS signal, the term xe2x80x9cZxe2x88x921xe2x80x9d of the denominator in this transfer function can be made zero. As a result, the frequency of the transfer zero point can be made coincident with the subcarrier of the ARI modulation signal. Consequently, there are no quantization errors specific to a digital filter, the ARI signal can be attenuated in high precision, and also the hardware size can be reduced.
Similarly, since the signal processing time period of the second infinite impulse response type filter is selected so that its frequency is higher than that of the subcarrier of the RDS signal, the term xe2x80x9cZxe2x88x921xe2x80x9d of the denominator in this transfer function can be made zero. Accordingly, the frequency of the pass band can be made coincident with the subcarrier of the ARI modulation signal. As a result, there are no quantization errors specific to a digital filter, the RDS signal can pass through the second infinite impulse response type filter in high precision, and the associated hardware size can be reduced.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.